The present invention relates to magnetically enhanced reactive ion etching (RIE) of substrates and, more particularly, to magnetically enhanced reactive ion etching of substrates of silicon wafers in bromine plasmas.
During fabrication of integrated circuits it is sometimes desirable to remove material from a wafer by etching processes. These etching processes can be characterized by their selectivity (i.e., ability to "attack" certain materials but not other materials) and by their anisotropy (i.e., ability to etch in one direction only, as opposed to undesired isotropic etching, in which material is etched in all directions at the same rate). Such etching processes can also be characterized by the spatial uniformity of the etch rate across the surface of the material to be etched.
Anisotropic etching of single-crystal and polycrystalline silicon with high selectivity to silicon dioxide has many important, recognized applications. One such application with respect to polycrystalline silicon (polysilicon) is in the area of metal-oxide-semiconductor field-effect transistor gate fabrication where oxides less than 10 nm thick must effectively stop an etch of up to 0.5 .mu.m of polysilicon. Etching of single-crystal silicon using a silicon dioxide mask also has important applications in fabricating trenches for device isolation, trench memory cells for dynamic random access memory, and channeling and grid-support masks for ion-beam lithography.
Plasma etching of surfaces such as silicon wafers is well-established technology. Plasma etching involves a chemical reaction whereby material to be removed undergoes conversion to a volatile state in the presence of at least one chemically active species produced in a gas discharge. Reactive ion etching (RIE) processes combine the use of chemically active species with ion bombardment. In RIE processes, it is believed that ion bombardment results in damage to the surface, which is then more easily removed by the chemically active species. It should be noted that there are other explanations for the RIE mechanism as well, involving, for example, ion induced chemical reactions. Thus, in a basic sense, RIE processes constitute an attempt to strike a desired balance between purely chemical and purely physical processes. The former (e.g., dissolution) is highly selective but isotropic, while the latter (e.g., bombardment with high-energy ions) is inherently isotropic but less selective.
The desire for uniformity of etching has led to various refinements of plasma etching systems. Some such refined systems involve "tunnel" reactor designs having multiple gas inlets. Other such refined systems involve "planar" reactors in which substrates are positioned on a planar electrode. In such systems, wafers are directly in the plasma so that energetic species having high recombination rates may be used. The wafers are positioned normal to the rf field so that ion movement is both rapid and highly directional.
There is growing recognition of the importance of low-energy etching processes because high-energy etching can cause significant lattice damage to etched surfaces and oxide breakdown during gate fabrication. Furthermore, it is known that selectivity to oxide can be increased at low ion energies. Various approaches are available for achieving high etch rates at low voltages. The most commonly used approach is high-pressure "plasma" etching. Although designated a low voltage system, operating voltages in such systems well exceed 110-V. The two other approaches known to those skilled in the art are designated "magnetron" and "flexible diode" systems.
Etching of silicon with chlorine- and fluorine-based plasmas in RIE reactors is well-known and common. In both cases, selectivity to oxide has been typically found to be less than 20. Chlorine-based etching is anisotropic in the RIE mode except for very high doping concentrations, although undercutting has been detected in the plasma etching mode, even for lightly doped material. Fluorine-based etching is intrinsically isotropic and photoresist etch rates are high. Oxygen or polymer-forming gases have been added to fluorine-based etching systems to achieve anisotropic etching by suppressing side-wall erosion. Addition of polymer-forming gases tends to reduce photoresist etch rates. But, systems containing polymer-forming gases often require a large amount of maintenance and have relatively low resolution.
Other types of plasmas have been used as well, including CF.sub.3 Br. When CF.sub.3 Br is used in RIE systems, highly anisotropic silicon profiles have been produced. However, undercutting has been detected in plasma etching of both doped and undoped polysilicon, with relatively poor oxide selectivity. Thus, CF.sub.3 Br is not wholly satisfactory.
Notwithstanding the many permutations of systems in the prior art, these systems have various shortcomings. None, for example, combines anisotropy for all levels of doping (n+), low resist etching rates, ultra-low silicon dioxide etching rates, ultra-high resolution, and low maintenance requirements. In addition, many of the prior art systems involve the use of multiple-gas plasma systems. Others result in low selectivity, anisotropy or uniformity. Because every system lacks one or more of these features, none is especially suitable for producing silicon stencil masks, for etching VLSI polysilicon gates, or for accomplishing other important applications.